Fix git-p4 rebase to detect the correct upstream branch instead of unconditionally

always rebasing on top of remotes/p4/master

git-p4

@@ -1361,9 +1361,17 @@ class P4Rebase(Command):
     def run(self, args):
         sync = P4Sync()
         sync.run([])
-        print "Rebasing the current branch"
+
+        [upstream, settings] = findUpstreamBranchPoint()
+        if len(upstream) == 0:
+            die("Cannot find upstream branchpoint for rebase")
+
+        # the branchpoint may be p4/foo~3, so strip off the parent
+        upstream = re.sub("~[0-9]+$", "", upstream)
+
+        print "Rebasing the current branch onto %s" % upstream
         oldHead = read_pipe("git rev-parse HEAD").strip()
-        system("git rebase p4")
+        system("git rebase %s" % upstream)
         system("git diff-tree --stat --summary -M %s HEAD" % oldHead)
         return True